I needed to install something on one of my home automation servers running Ubuntu 19.04, and suddenly couldn’t - lots of errors with Disco updates and packages not found:
What’s the problem with updates in Ubuntu 19.04?
Turns out, 19.04 is too old now - so it’s been officially migrated from archive to old-releases repos in Ubuntu.
How To Fix The Missing Ubuntu 19.04 Updates
You need to edit the /etc/apt/sources.list file. Update the following lines:
Aftet this apt update will work just fine: